T-Mobile may have for some time a power failure to complain, where it it no longer possible for smartphone and mobile phone users to log on to the mobile network. Thus no telephone calls could be led or accepted and also the mobile Internet could not be used.
Anyone who is affected by such a problem or will be affected in the future, we would like to give you a tip if no connection can be established. Often it helps if you switch the smartphone once in the airplane mode and deactivate it after about 30 seconds.
Strangely, this approach helps to log back into the mobile network faster. Background in this case was that the Telekom's servers did not recognize the SIM card profiles correctly. By activating and deactivating the aircraft mode, a new request was sent to the mobile network. By sending a request more often, the probability was greater that the sim card profile was recognized at some point.
As we said, we recommend you this tip for future events.